1. Field of Invention
This invention relates generally to excavation and more particularly to excavation using air jets to loosen soil and to using vacuum for removing soil from an excavation face.
2. Description of the Related Art
Trench digging is a common activity for installing or gaining access to buried utilities such as electric power cables, natural gas piping, communications cables, and water pipes. The backhoe is a traditional trencher which uses an upside-down bucket mounted on a hinged boom. A trenching machine uses a series of buckets mounted on a chain or wheel to lift soil from the ground and to deposit that soil alongside the trench to be dug. The hand-wielded pick or shovel provide a relatively low capacity trenching means.
In excavation, there are two principal actions involved--soil cutting and soil removal. In trenching that uses conventional equipment such as that described above, hard cutting teeth or cutting edges are forced into the ground to "cut" the soil. This equipment can not only cause damage to buried utilities but it can also expose itself (the equipment) and personnel who operate it to severe hazards associated with damaged utilities. These hazards include electrocution, natural gas explosion, and water damage.
Another drawback associated with the above trenching equipment is a lack of continuity in soil cutting and soil removal. For example, a backhoe first loads its bucket which is the soil cutting phase of its operation. When the bucket is full, soil cutting must be suspended while the bucket is lifted from the excavation face to dump soil onto a pile or into a removal vehicle. This suspension of the soil cutting phase slows the potential production rate. Trenching machines attempt to provide more continuous cutting and removal. However, they still have destructive hard cutting edges or cutting teeth. Trenching machines have such a high damage potential that they are not used in locales where there is a likelihood of buried utilities.
The idea of a "soft excavator" is to use jets of air to loosen soil and the like without damage to utility lines or to other "hard" buried objects. A conventional air jet cutting device which loosens soil but which does not provide an effective means for removing loosened soil from a trench addresses only one of the two principal actions involved in excavation. As will be discussed in the following two paragraphs, a conventional soft excavator which uses a flexible hose for soil transport or which uses a tank for accumulating and holding soil has important imitations.
A soft excavator which uses a flexible hose for the transport of soil has limited throughput capacity because of the adhesive and cohesive nature of soils. Some flexible hoses, especially those for vacuum service, have a reinforcing spiral spring that forms ridges inside the hose and that can cause soil to stick to the sides of the interior of the hose. This loss of transport pipe area results in an increased pressure drop in the pipe and in less effective soil transport. Additional particles impact the sidewalls and adhere, thereby further reducing the transport area to the point where clogs could occur. The bends associated with flexible hoses also contribute to a propensity of flexible hoses to clog. Clogging is a major problem for prior art soft excavators.
Prior art soft excavators which use a tank or hopper for accumulating and holding excavated material also have limited throughput capacity because the cutting action of the excavator must be periodically stopped to permit the tank or hopper to be emptied.